How Deep Should Footings Be?

Field Workflow Before Pouring

1. Confirm local frost depth and permit requirements.
2. Excavate to plan depth.
3. Inspect the bearing surface for soft zones or fill.
4. Measure footing width, depth, and length for concrete volume.
5. Do not place concrete until inspection is complete where required.

Field verification of footing depth, step by step

1. Strip organics to mineral soil. All grass, topsoil, and root mat must come out before depth measurement starts. Anything less than mineral soil doesn't count toward footing depth.
2. Measure from finished grade, not from the bottom of the excavation. "36 in deep" means 36 in below where the finished slab or backfill will be, not 36 in below the ground level before digging started.
3. Verify bearing soil by probing. A 5/8 in rod driven with a 10 lb drop hammer should refuse in competent soil within a few blows. Many blows = soft soil = deeper footing needed.
4. Confirm dry conditions. If water is standing in the footing excavation, either pump it out and wait 4 hours to see if it returns, or design for the wet condition with a deeper footing.
5. Check the footing width against the load. IRC Table R403.1 gives minimum widths: 16 in for 1-story, 21 in for 2-story on soil with 1,500 psf bearing capacity. A 16 in footing on a 2-story load is overstressed.
6. Install drain tile if water is a concern. A 4 in perforated drain pipe in gravel at the footing level, sloped to daylight, eliminates hydrostatic pressure. Cost: roughly $2 to $3 per linear foot installed.
7. Photograph the footing depth with a tape in frame. Before the concrete is placed. This photo is your evidence if the inspector questions depth later.

On a deck rebuild project in Pennsylvania in 2022, I found the original footings at 26 in depth in a 42 in frost zone. The deck had been moving 2 in up in winter and dropping back in spring for 11 years. The ledger board was actually pulling away from the house, a movement measurable in tenths of an inch per year. We rebuilt the footings to 48 in, reflashed the ledger, and the deck has been stable since. Depth is not optional when frost is in the equation.

Depth Is a Code Minimum, Not a Soil Investigation

When I review pier footings, strip footings, sonotubes, gravel pads, and bearing soil on a job, I treat the published rule as the starting point, not the finished answer. The missing layer is the field condition: moisture, compaction, soil behavior, delivery tolerance, and the specific code table that applies in that county. In a Pennsylvania deck where 26 in footings in a 42 in frost zone moved enough to pull the ledger flashing open, the calculator math was not the problem. The problem was that nobody translated the calculator output into a field-controlled specification.

The checks below are the ones I use before I approve an order or a layout. They are deliberately numeric because vague wording such as "good gravel," "deep enough," or "standard slope" is where residential projects lose money. If the number is written down, a supplier, inspector, or crew lead can challenge it before material is placed. If the number is only assumed, the mistake usually shows up after the truck has left.

• 30 in Delaware frost
• 42 in Pennsylvania frost
• 1,500 psf soil
• 16 in footing width
• 6 in below frost

The recurring risk is digging to frost depth but stopping in soft organic soil. My field correction is simple: probe the bottom of the hole and verify competent mineral bearing before pouring. This is a small step, but it creates a paper trail and a repeatable decision. It also gives the homeowner a fair way to compare bids. A bid that includes density, compaction, depth, or code reference is usually more reliable than a cheaper bid with only a lump sum.

I also price the cost of being wrong. On one recent job, $180 of extra excavation prevented a $6,500 underpinning repair. That is the kind of practical difference a guide page should help you catch before you call the supplier. The calculator gives the quantity; the field check protects the quantity from becoming the wrong purchase.

Sarah's pre-order verification notes

1. Write down the assumed density, depth, spacing, or slope. I do not let a number remain implied. If it drives cost, it belongs on the order sheet.
2. Confirm the unit with the supplier or inspector. Feet, inches, cubic yards, tons, percent slope, and ratios are all easy to mix when a quote moves from phone call to invoice.
3. Check the tolerance. I allow 5% on simple rectangular material orders, 10% on irregular shapes, and 15% when curved edges, wet material, or compacted volume are involved.
4. Photograph the condition before covering it. A photo of a tape measure in a footing, a delivery ticket next to a stone pile, or a laser reading on a slope has settled more disputes for me than any email thread.
5. Do one reverse calculation. Convert the final order back into area, depth, or load. If the reverse answer does not match the site sketch, the order is not ready.

That five-step habit is not glamorous, but it is how I keep small residential jobs from developing commercial-sized change orders. We have measured the same pattern across driveways, patios, decks, grading work, and concrete pours: the expensive mistake is usually visible in the numbers before it is visible in the finished work.